The steady drumbeat of headlines heralding the rise of the superbugs might seem a bit sensationalistic. But it’s not just puff and fear-mongering — superbugs are here, and they are a real risk. They’re also deeply confusing. That’s because superbugs are more than one thing. The word can mean a specific microorganism that has developed resistance to antibiotics — that’s what was happening when media warned of the growing threat of antibiotic-resistant gonorrhea in 2011. It can also mean a specific mechanism that creates antibiotic resistance and that might be shared by several different microorganisms — as in the most recent news. There are many microorganisms, many antimicrobials and many kinds of mechanisms that confer resistance. And so there are plenty of times that superbugs can arrive in the U.S. for the first time.

The news from last week — which that NBC News headline covered — is a great example of this. It was about a mechanism of resistance, MCR-1, that was discovered in a sample of E. coli bacteria taken from a Pennsylvania woman’s urinary tract infection. It was the first time MCR-1 has been found in the United States. MCR-1 is a plasmid, which is basically a second batch of DNA, distinct from a bacterium’s own chromosomal DNA, that can replicate independently. “This means that resistance spreads because the plasmid can move from one bacteria to another,” Jean Patel, deputy director of the CDC’s office of antimicrobial resistance, wrote in an email. So you have bacteria spreading from person to person, as diseases do. Then you have plasmids spreading from bacteria to bacteria, almost like their own little epidemic, and jumping species as they go.

Adding to the confusion — and the growing sense of unease — is the fact that some of the bacterial samples that have contained MCR-1 were archival. They date back years, even decades in China’s case, meaning MCR-1 existed in some countries before scientists realized it did.

That’s probably true for the U.S. That’s because the bacteria that carry it aren’t always making people sick. Globally, the samples of bacteria containing MCR-1 have come from humans, but also from animals, food and streams. Even in humans, infection with MCR-1-carrying E. coli doesn’t mean you get sick. We carry E. coli around with us in our intestines every day. Not all E. coli are bad for humans. If a friendly E. coli living peacefully in your intestines carries MCR-1, you’d never know. The plasmid only becomes important if that E. coli makes you sick and your doctor tries to kill it — and can’t. This reality makes it difficult for scientists to track the spread of MCR-1 through bacterial populations. And it makes it easier for MCR-1 to spread. That’s because those silent reservoirs of MCR-1-carrying E. coli are also home to other bacteria, both different types of E. coli and different species. The more contact those bacteria have, the more opportunities MCR-1 has to jump into something that can make you sick.

When dangerous bacteria become resistant, evidence shows they are, indeed, a serious threat. But the most deadly isn’t always the most dangerous. When the CDC — which tracks deaths by bacteria, not resistance factor — ranks microbial dangers, it considers not just how many Americans the antibiotic-resistant bugs kill each year, but also how quickly and easily the bugs spread, how many drugs remain that can kill them, and more.

CLASS OF RESISTANT BACTERIA

CDC THREAT LEVEL

EST. ANNUAL INFECTIONS

FATALITY RATE

Carbapenem-resistant Enterobacteriaceae (CREs)

Urgent

9k

6.6%

Clostridium difficile infections

Urgent

250

5.6

Drug-resistant gonorrhea

Urgent

246

<0.1

Methicillin-resistant Staphylococcus aureus (MRSA)

Serious

80

13.8

Multidrug-resistant Acinetobacter

Serious

7

6.8

Multidrug-resistant Pseudomonas aeruginosa

Serious

7

6.6

Extended-spectrum β-lactamase producing Enterobacteriaceae (ESBLs)

Serious

26

6.5

Vancomycin-resistant Enterococcus (VRE)

Serious

20

6.5

Drug-resistant Candida

Serious

3

6.5

Drug-resistant tuberculosis

Serious

1

5.0

Streptococcus pneumoniae

Serious

1,200

0.6

Drug-resistant Salmonella typhi

Serious

4

0.1

Drug-resistant nontyphoidal Salmonella

Serious

100

<0.1

Drug-resistant Shigella

Serious

27

<0.1

Drug-resistant Campylobacter

Serious

310

<0.1

Erythromycin-resistant Group A Streptococcus

Concerning

1

12.3

Clindamycin-resistant Group B Streptococcus

Concerning

8

5.8

CDC threat rankings of antibiotic-resistant bacteria in the U.S.

Source: Centers for Disease Control and Prevention

For instance, the CDC estimates that methicillin-resistant Staphylococcus aureus, or MRSA — which has been in the news for infecting NFL players and even costing some their careers — infects 80,000 Americans annually and kills 11,000 of them. But it’s considered less of an immediate risk than carbapenem-resistant Enterobacteriaceae (or CREs), which kill only about 600 people annually. That’s because these bacteria can shrug off almost any antibiotic you throw at them. The CDC says that E. coli carrying MCR-1 would be analogous to CREs. They haven’t been directly responsible for many deaths yet, that we know of. But because we have nothing that can kill them, people who do get infected are in big trouble.

You’ll likely see more firsts for MCR-1 in the coming months and years. Case in point: The very same day the Pennsylvania woman’s case was announced, the U.S. Department of Health and Human Services announced the country’s first animal case of MCR-1 E. coli, in the intestine of a slaughtered pig.

Maggie Koerth-Baker is a senior science writer for FiveThirtyEight. @maggiekb1